This invention relates generally to a projection-type color display device, and more particularly to the driving circuitry for liquid crystal light valves used in a projection-type display device to project and form a symmetrical colored image on a screen.
Conventional projection-type color display devices which use liquid crystal light valves such as disclosed in Japanese Laid Open Application No. 60-179,723 initially separate the image to be displayed into its red, green and blue wavelengths of light using a dichroic mirror. Each of these wavelengths of light is then directed toward a corresponding light valve which permits or prevents transmission of light therethrough (i.e., image modulation). The red, green and blue wavelengths of light which are permitted to pass through these light valves are synthesized by additive color mixing using dichroic mirrors, a dichroic mirror prism or the like. The synthesized image is then magnified and projected onto a screen through a projection lens.
Light sources used in projection-type color display devices are unable to project perfectly parallel rays of light. The intensity of these nonparallel rays of light, especially as the distance from the light source increases, diminishes (i.e., dampens). Consequently, the brightness of the image produced by the device diminishes.
Spectral characteristics of the light source are not constant, that is, the spectral characteristic curve is not flat. A neutral density (ND) filter is typically used to adjust the white balance of the light source. The ND filter dampens the peaks of the spectral characteristic curve. Depending upon the light source, a ND filter may be unable to provide an acceptable white balance. For example, a metal halide lamp has a spectral characteristic curve with a peak around the green wavelength. Dampening of the green light through use of an ND filter results in extremely dark images formed by a conventional projection-type color display device.
U.S. patent application Ser. No. 07/181,122, discloses that each of the three liquid crystal light valves include a matrix array of picture element electrodes coupled to a corresponding thin film transistors (referred to hereinafter as thin film transistors (TFT)) to control which picture element electrodes are turned ON and OFF. The light sensitive TFT's are shielded from incident light passing through the light valve to avoid generation of unnecessary leakage current by a mask (light protection layer) formed on one of the substrates of each light valve.
The three liquid crystal light valves within the projection-type color display are arranged so that one of the three light valves is reversed (i.e. flipped) relative to the other two light valves to provide a physically compact device. This is done to allow scanning horizontally in the opposite direction. However, flipping the light valve allows incident light to strike the TFTs before passing through the associated mask. The mask fails to shield the TFT from the incident light and the performance of the TFT is adversely affected. An unacceptable contrast ratio in the image and overall deterioration in the picture quality results.
Accordingly, it is desirable to provide a projection-type display using liquid crystal light valves which are assembled in a compact image display device. The device should provide bright images with a high level of picture quality and a high level of picture reproducibility. It is also desirable to provide a projection-type color display device which avoids the use of one or more filters to adjust the white balance of the light source.